The cortex of the roots of a susceptible and a resistant variety of Brassica campestris var. rapa infected with sterile resting spores of Plasmodiophora brassicae from senescent callus was studied at a stage prior to disease symptom development. Electron micrographs show the presence of amoeboid structures within the cortical cells of the susceptible variety 10 days after inoculation. Cell wall perforations, hypertrophied host cell nuclei, nucleoli and broken tonoplasts were frequently found in the susceptible variety. It has been concluded that amoeboid structures of the parasite penetrate the cell wall and disrupt the cortical cells.Electron micrographs of the resistant variety show the presence of zoosporangia with secondary zoospores in the root hairs nine days after inoculation. Two to four days later a large number of dead host cells can be observed in the outer cortical layer of the resistant variety, whereas no apparent changes are found in the inner cortex. The results suggest the occurrence of a hypersensitive host reaction which terminates further growth ofPlasmodiophora brassicae.

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This paper deals with the quantitative determination of free and bound cytokinins in clubroot tissue and in Flasmodiophora brassicae Woron. infected Brassica campesiris L, callus tissue. The fractions were separated in a butanol soluble fraction containing the free cytokinins such as zeatin and zeatin riboside and a water soluble fraction containing the bound cytokinins.The butanol fraction was extensively purified and analysed by high pressure liquid chromatography (HPLC), The butanol fraction contained cytokinins which cochromatographed with zeatin and zeatin riboside and not with dihydrozeatin, Zeatin and zeatin riboside were quantitatively determined by HPLC, Recovery of the cytokinins varied between 30-50%, Clubs contained 50-160 ng zeatin and 210-300 ng zeatin riboside per g dry weight. Callus tissue contained 133 ng zeatin and 169 ng zeatin riboside per g dry weight.Clubs, callus as well as healthy tissue contain large amounts of bound cytokinins. Upon treatment of the water soluble fraction first with alkaline phosphatase and then with {i-glucosidase biologically active fractions were found which coeluted with zeatin and zeatin riboside on Sephadex LH2(i in 20% ethanol. Evidence is presented for a novel cytokinin in the water soluble fraction which yields free zeatin and glucose-6-phosphate after treatment with P-glucosidase,

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